The present invention relates generally to filtration of fluids, including especially filtration of water.
There is a great worldwide demand for purified fluids, one of the most commercially important of which is production of fresh water. Many areas of the world have insufficient fresh water for drinking or agricultural uses, and in other areas where plentiful supplies of fresh water exist, the water is often polluted with chemical or biological contaminants, metal ions and the like. There is also a continuing need for commercial purification of other fluids such as industrial chemicals and food juices. U.S. Pat. No. 4,759,850, for example, discusses the use of reverse osmosis for removing alcohols from hydrocarbons in the additional presence of ethers, and U.S. Pat. No. 4,959,237 discusses the use of reverse osmosis for orange juice.
Aside from distillation techniques, purification of water and other fluids is commonly satisfied by filtration. There are many types of filtration, including reverse osmosis (RO), ultra-filtration and hyper-filtration, and all such technologies are contemplated herein within the generic term, xe2x80x9cfiltration.xe2x80x9d
Reverse osmosis involves separation of constituents under pressure using a semi-permeable membrane. As used herein, the term membrane refers to a functional filtering unit, and may include one or more semi-permeable layers and one or more support layers. Depending on the fineness of the membrane employed, reverse osmosis can remove particles varying in size from the macro-molecular to the microscopic, and modern reverse osmosis units are capable of removing particles, bacteria, spores, viruses and even ions such as Clxe2x88x92 or Ca++.
There are several problems associated with reverse osmosis (RO), including excessive fouling of the membranes and high costs associated with producing the required pressure across the membranes. These two problems are interrelated in that most or all of the known RO units require flushing of the membranes during operation with a relatively large amount of feed liquid relative to the amount of permeate produced. The ratio of flushing liquid reject to permeate recovery in sea water desalination, for example, is about 3:2. Because only some of the sea water being utilized is recovered as purified water, energy used to remaining water is wasted, creating an inherent inefficiency.
It is known to mitigate the energy cost of filtration pumping by employing a work exchange pump such as that described in U.S. Pat. No. 3,489,159 to Cheng et al. (January 1970) which is incorporated herein by reference. In such systems, pressure in the xe2x80x9cwastexe2x80x9d fluid that flows past the filter elements is used to pressurize the feed fluid. Unfortunately, work exchange pumps employ relatively complicated piping, and in any event are discontinuous in their operation. These factors add greatly to the overall cost of installation and operation.
It is also known to mitigate the energy cost of filtration pumping by employing one or more turbines to recover energy contained in the xe2x80x9cwastexe2x80x9d fluid. A typical example is included as FIG. 3 in PCT/ES96/00078 to Vanquez-Figueroa (publ. October 1996), which is also incorporated herein by reference. In that example, a feed fluid is pumped up a mountainside, allowed to flow into a filtration unit partway down the mountain, and the waste fluid is run through a turbine to recover some of the pumping energy.
A more generalized schematic of a prior art filtration system employing an energy recovery turbine is shown in FIG. 1. There a filtration system 10 generally comprises a pump 20, a plurality of parallel permeators 30, an energy recovery turbine 40, and a permeate or filtered fluid holding tank 50. The fluid feed lines are straightforward, with an intake line (not shown) carrying a feed fluid from a pretreatment device (not shown) to the pump 20, a feed fluid line 22 conveying pressurized feed fluid from the pump 20 to the permeators 30, a permeate collection line 32 conveying depressurized permeate from the permeators 30 to the holding tank 50, a waste fluid collection line 34 conveying pressurized waste fluid from the permeators 30 to the energy recovery turbine 40, and a waste fluid discharge line 42 conveying depressurized waste fluid from the energy recovery turbine 40 away from the system 10.
A system according to FIG. 1 may be relatively energy efficient, but is still somewhat complicated from a piping standpoint. Among other things, each permeator 30 has at least three pressure connectionsxe2x80x94one for the feed fluid, one for the waste fluid, and one for the permeate. In a large system such fluid connections may be expensive to maintain, especially where filtration elements in the permeators need to be replaced every few years.
WIPO publication 98/46338 discloses an improvement in which production modules containing spiral wound filters are mechanically coupled in series, while providing the feed, filtered, and waste fluid flowpaths in parallel. This arrangement, sometimes referred to herein as S/P modularization, allows a series of coupled modules to be conveniently installed, accessed, and removed. WO 98/46338 suggests numerous ways of deploying the modules in space efficient manner, such as by insertion into a deep or shallow well, a tower, along the ground, into the side of a hill or mountain, or even under a road or parking lot. It is also suggested that efficiency in installation and removal can be enhanced by mating adjacent production modules with one another using a slip fit joint, and that the production modules may be maintained in mating relationship through connections to supporting cables or rods.
WIPO Publication WO 98/46338 did not, however, teach mitigating the energy cost of S/P modularized production modules by employing a modularized energy recovery device. Thus, there is a continuing need for a simplified approach to recovering energy costs employed in such systems, especially in large scale systems (at least 1 million gallons per day) in which inefficiencies can be very significant.
The present invention is directed to modularized filtration systems having an energy recovery device. In preferred embodiments the filtration system is arranged so that multiple production modules mechanically are coupled in series to form a production chain, and each of a common feed fluid flowpath, a common waste fluid flowpath, and a common product flowpath carried along the production chain. In yet another aspect of preferred embodiments the energy recovery device comprises a turbine positioned to extract energy from a flow-by or xe2x80x9cwastexe2x80x9d fluid. Still more preferred embodiments additionally include a modularized pressurization device for pressurizing a feed fluid, and provide a common drive shaft for the energy recovery and pressurization devices.